As a computer technician myself, I can recommend you give this idea a try. Yes, unfortunately what Drapony said is sort of true, I'm afraid, but the market still exists. And you can always highlight the advantages of having a desktop to your clients! It's a neat way to make some extra cash but above it all, it's a fun thing to do. I personally like to build computers. If you like it as well, make some profit out of it should be definitely welcome.
what part am I wrong about, just curious ^>^
I said "sort of true" because, though it is true that many people are choosing laptops/tablets more often instead of actual computers (desktops), most are doing it out of ignorance. They see these small devices and because they don't know any better, they fall for the advertising of the companies that promise these things can do absolutely everything a good ol' PC can do (sometimes even better) but with the possibility of occupying less space, being lighter, possible to carry around, etc. Many of them later on end up realizing it's not all sunshine and flowers in the world of portable/mobile, though, and eventually end up coming back to desktop computing. This is the reason why often you find people in online games complaining they have a crappy computer to play and it can't handle the game in question well. If you ask them, most will tell you they are playing on a laptop.
That isn't true, a lot of people want desktops, but the computer companies are trying to faze the affordable desktop out cause Laptops make them more money, since when something in a laptop brakes you can't just fix it yourself, and often have to get a new laptop all together. So you could probably make good money offering people affordable custom desktops and fill the nesh the computer companies are neglecting <3